Orthodontic tooth movement in mechanotherapy depends on the ability of a clinician to use controlled mechanical forces to stimulate biologic responses within the paradental tissues surrounding the roots of treated teeth. The most common orthodontic technique consists of an edgewise bracket that slides along an archwire. The sliding arch-guided system, or sliding mechanics, can be influenced by counteracting frictional forces at the interface of the bracket, archwire, and ligature. Thus, the basic principle of orthodontic appliances in the continuous archwire technique is to apply mechanical forces on teeth so that movement will occur in every desirable spatial direction. The forces are applied during the various stages of treatment by a variety of appliances, which include several kinds of archwire, ligatures, and bands.
Friction among appliances used for orthodontic correction of teeth is recognized by clinicians as a hindrance to tooth movement. Friction reduces the effective force, which is applied to the tooth from the wire. In the case of sliding mechanics, excessive friction, brought about by the angle between the wire and the slot of the bracket, slows tooth movement down substantially or even halts it. It would be of great clinical advantage to minimize this friction so that the clinician can accurately estimate the amount of force transmitted to each tooth. Unimpeded tooth movement would reduce the time needed for treatment, thereby reducing the risks of adverse effects of wearing orthodontic braces on the teeth and surrounding tissues. It would be desirable to minimize the effect of any factors over which a clinician has no control, including the amount of friction between the archwire and the bracket.
There are several variables that affect the amount of friction between orthodontic appliances, most prevalent of which is the composition of the wire and bracket material. The problem of friction is compounded by the desire for aesthetically pleasing appliances such as ceramic brackets, which have greater friction toward all types of archwires than stainless steel brackets. Archwires made of beta-titanium have desirable properties such as good flexibility, spring force and biocompatibility, and are the only orthodontic wires containing no nickel. However, these materials are soft, have poor frictional characteristics and are very susceptible to adhesive wear. Orthodontic tooth movement in mechanotherapy depends on the ability of a clinician to use controlled mechanical forces to stimulate biologic responses within the tissues surrounding the roots of treated teeth, most commonly by way of an edgewise bracket sliding along an archwire. Frictional forces at the interface of the bracket, archwire, and ligature can hinder these sliding mechanics.
The friction coefficients between bracket and various wire materials have been measured extensively. Wires made out of such popular shape memory materials such as titanium-molybdenum alloy (TMA) and Ni--Ti (nickel-titanium) alloys tend to be softer than stainless steel and tend to have higher coefficients of friction against stainless steel than does stainless steel itself, which has a high coefficient of friction. This is true particularly at high (.about.80 N) normal forces.
Coating thin films of various materials onto archwires has been previously suggested as a way to reduce friction and to improve their aesthetic appearance. U.S. Pat. No. 5,288,230 to Nikutowski et al. describes applying a coating of diamond-like carbon (DLC) onto archwires to serve as a barrier to diffusion of nickel and chromium from the wire, which causes allergic reactions in the patient. The DLC coatings also provide a hard, friction-reducing surface. The appearance of the DLC films, however, ranges from black to interference rainbow colors, and is not aesthetically pleasing. U.S. Pat. No. 5,454,716 to Banerjee et al. describes a coating of a plastic-ceramic composite, which is aesthetically pleasing, but is susceptible to localized abrasion over time.
Ion implantation has been used to modify orthodontic materials with the goal of reducing friction. For example, alumina, which is a commonly used ceramic in bracket manufacture, has been implanted with titanium with a resulting decrease in its coefficient of friction with beta-titanium wire. (Kusy et al., Dental Material 8:167-172, 1992). Friction reduction was observed for alumina flats coated with DLC. (R. P. Kusy et al., (1993) J. Am. Ceram. Soc., 76[2], 336-342 Implantation of beta-titanium wires with nitrogen reduced the friction due to formation of hard titanium oxy-nitride on the wire. (R. P. Kusy et al., Nanda, 215-221 (1993); R. P. Kusy et al., Dental Mater., 8, 167-172 (1992); R. P. Kusy et al., (1993) J. Am. Ceram. Soc., 76[2], 336-342).